Field
This invention relates generally to a thermal calibrator for calibrating a camera and, more particularly, to a thermal calibrator for calibrating a passive millimeter wave (PMMW) camera, where the calibrator includes two thermally conducting blocks separated by a thermo-electric (TE) cooling device that cools one of the blocks and heats the other block to provide calibration targets at hot and cold temperatures, and where the target is selectively scanned across a focal plane array (FPA) in the camera.
Discussion
PMMW cameras are well known in the art that passively receive and process millimeter wave radiation from a scene and provide imaging through thermal resolution of objects in the scene. Certain millimeter wave frequencies in the millimeter wave radiation band of 20-300 GHz, such as 35 GHz, 94 GHz, 140 GHz and 220 GHz, are not significantly attenuated by smoke, fog, clouds, etc. in the air, and thus provide radiation that can be detected for scene imaging purposes when visible light imaging cannot be used. For example, aircraft can employ PMMW cameras to detect runways through clouds, smoke and fog.
A typical PMMW camera that detects and images radiation in these frequency bands often includes a focal plane array (FPA) that converts the radiation into an electric signal, where a lens focuses the radiation onto the array. The FPA typically includes a configuration of a plurality of receivers positioned in a two-dimensional plane, where each of the receivers includes an antenna or signal horn having a pick-up probe at the front end that converts the radiation to an electrical signal that is amplified by a millimeter integrated circuit (MMIC) low noise amplifier. A diode at the back end of the each receiver rectifies the amplified voltage signal to a DC signal, where the DC signal amplitude is representative of the power level of the received signal, which increases as the radiometric temperature of the object being imaged increases, and where power and temperature are proportional to each other. The DC voltage signal from each receiver is then digitized and converted to an image, where higher voltages are displayed as whiter areas in the image representing warmer objects with higher radiometric temperature.
The relationship of the receiver DC signal voltage Voutput to the scene temperature Tscene is given by equation (1) below. The calibration involves obtaining a gain and offset value for each receiver that allows the conversion of the receiver output voltage into a scene radiometric temperature that can be converted into a gray scale for display. Particularly, the gain and DC offset of each receiver are typically not the same. If a receiver views two calibration targets having temperatures T1 and T2, and produces output voltages V1 and V2, respectively, the associated receiver gain and offsets are defined in equations (2) and (3), respectively.Voutput=GAIN*Tscene+Voffset,  (1)where:GAIN=(V2−V1)/(T2−T1),  (2)Voffset=V1GAIN*T1.  (3)
Therefore, a calibration technique is required to calibrate each of the receivers so that they provide the same voltage level for the same power of the incoming radiation. Known calibration techniques for PMMW receivers include placing a black body target, such as a target that is cooled to the temperature of liquid nitrogen, in front of the FPA that is representative of the coldest value that the receivers could receive and then identifying the specific voltage level for each receiver for that target temperature so that each receiver has a calibration factor for the low temperature. The calibration technique also includes placing a warm target in front of the FPA, such as at room temperature, and providing a calibration factor for each DC voltage level provided by each receiver for the warm target temperature.
Each receiver will respond to different signal power levels in a linear manner so that a line between the cold calibration target temperature point and the hot calibration target temperature point represents a calibration curve for that receiver. The gain and offset for each receiver is then determined from this calibration curve, as defined by equations (2) and (3). This process for calibrating the receivers in an FPA would need to be performed as often as required depending on the drift of the receivers in the array. The FPA drift may be a result of temperature changes in the environment, and the calibration curve for each receiver would need to be updated for the new receiver temperature. For example, if the camera is mounted on an aircraft, it will experience significant temperature changes for different altitudes of the aircraft.
These known calibration techniques that require hot and cold targets sometimes require that these targets be mechanically moved in front of the FPA during the calibration process. For a PMMW camera with a large field-of-view, an FPA operating in the MMW, with elements placed in a Cartesian arrangement, can be on the order of 10 cm×10 cm. Thus, providing two targets having the same size as the FPA that need to be selectively and independently moved in front of the FPA during calibration and then moved away from the FPA during detection and imaging requires size and space requirements that are somewhat prohibitive.